1. Field of the Invention
This invention relates generally to X-ray generator systems and is concerned more particularly with X-ray tube control circuitry having means for indicating prior to operation of the tube the actual voltage applied between electrodes of the tube when the tube is operated.
2. Discussion of the Prior Art
A conventional X-ray tube type of X-ray generator system generally includes control circuitry having a polarized source of high voltage, such as the rectified output of a high voltage transformer, for example, disposed for applying a suitable high voltage between the cathode and anode electrodes of the X-ray tube. In operation, electrons emitted from the cathode are beamed electrostatically onto a focal spot area of an anode target with sufficient energy to generate X-rays which emanate from the tube in a beam. The maximum energy of X-rays in the beam is proportional to the maximum kinetic energy attained by the beamed electrons, which is a function of the voltage applied between the cathode and anode electrodes during operation of the tube.
In diagnostic radiology, for example, the X-ray beam may be directed through a selected portion of a patient and onto an aligned film to produce an X-ray shadow image of internal body structure in the selected portion. Consequently, if the selected portion is comprised of fleshy tissue, the voltage applied between the cathode and anode electrodes of the X-ray tube is relatively low in value in order to generate correspondingly low energy or "soft" X-rays. The soft X-rays have sufficient energy to penetrate through the fleshy tissue in the selected portion and produce on the aligned film an X-ray image having the desired resolution and contrast for defining detail structure, such as small blood vessels, for example. Conversely, if the selected portion is comprised of bony structure, the voltage applied between the cathode and anode electrodes is relatively high in value in order to generate correspondingly high energy or "hard" X-rays. The hard X-rays have sufficient energy to penetrate through the bony structure and produce on the aligned film an X-ray image having the necessary resolution and contrast for showing fine details, such as hairline cracks, for example.
Consequently, X-ray tube control circuitry of the prior art generally is provided with means for selecting and indicating the desired voltage to be applied between the cathode and anode electrodes of the tube during a planned X-ray exposure. The voltage selecting and indicating means of the prior art preferably is disposed for connection to the input primary rather than the output secondary of the high voltage transformer in order to avoid the high voltages applied between the cathode and anode electrodes during operation of the tube. However, it may be found that when an X-ray exposure is initiated there is a considerable drop in voltage which causes the actual voltage applied between the cathode and anode electrodes to be substantially less than the desired voltage. As a result, the X-rays generated may not have sufficient energy to penetrate through the selected portion of the patient and produce an X-ray image of sufficient clarity to form the basis of an accurate diagnosis of the patient's condition. Thus, the selected value of voltage may have to be increased; and the patient may be required to undergo additional X-ray exposures until the proper value of voltage less the voltage drop is found for producing a clear X-ray image of the internal body structure. These additional X-ray exposures generally are contrary to the requirements of public health laws which are intended to protect the patient from exposure to excessive X-radiation.